Many wireless communication systems use eNBs (base stations, eNodeBs, eNBs) to provide geographical service areas where wireless communication user equipment devices (UE devices) communicate with the eNB providing the particular geographical service area in which the UE devices are located. The eNBs are connected within a network allowing communication links to be made between the wireless communication devices and other devices. In some circumstances, the communication links are between wireless communication UE devices that are close to each other. In these situations, it may be preferred to have a direct communication link between the two wireless UE devices rather than communicating through a base station. Such direct communication between devices is often referred to as device-to-device (D2D) communication or peer-to-peer (P2P) communication. The communication resources (e.g., time-frequency blocks) used for D2D communication are typically a subset of the communication resources used by the communication system for communication between UE devices and the eNBs.
An in-coverage UE device (InC UE device) is a UE device that is within the service area of an eNB and is capable of communication with the eNB. An out-of-coverage UE device (OoC UE device) is a UE device that is not within a service area of any eNB. D2D UE devices that are engaged in D2D communication with each other form a D2D group. A D2D group, therefore, includes two or more D2D UE devices. There are several coverage scenarios that may occur with D2D groups. In an InC coverage scenario, all the UE devices of the D2D group are located in a service area of a single eNB. For the InC scenario, therefore, all of the D2D UE devices of the D2D group are InC UE devices in a single service area. In an OoC coverage scenario, none of the UE devices of the D2D group are located inside any service area of any eNB. For such a scenario, therefore, all of the D2D UE devices of the D2D group are OoC UE devices. In a partial coverage scenario, at least one of the UE devices of the D2D group is located in a service area of a single eNB and at least one D2D UE device of the group is outside all service areas. For the partial coverage scenario, therefore, at least one of the D2D UE devices of the D2D group is an InC UE device in a single service area and at least one UE device is an OoC UE device. Other coverage scenarios are possible.
In order to maintain synchronization of communication within a D2D group, a synchronization signal is typically received by the UE devices in the group. In some situations, the synchronization signal is transmitted by the eNB. Such a situation occurs where all the D2D UE devices are in coverage. In some situations, a D2D synchronization signal (D2DSS) is transmitted by one or more of the D2D UE devices in the D2D group to maintain synchronization. When available to the D2DSS transmitting D2D UE device, the D2DSS is based on system timing obtained directly from the eNB. In some situations, however, the UE device cannot obtain the timing information directly from the eNB and the D2DSS is not based on timing directly obtained from the eNB. Proposals for 3GPP LTE revisions include a mechanism for indicating whether a D2DSS is based on timing directly obtained from the eNB or not.